1. Field of the Invention
The present invention relates to an overvoltage protection circuit, an LED backlight driving circuit comprising the overvoltage protection circuit, and a liquid crystal display (LCD) having the LED backlight driving circuit.
2. Description of the Prior Art
With technical development, a backlight technology of a liquid crystal display has been developing. A conventional liquid crystal display uses cold cathode fluorescence lamp (CCFL) as a backlight source. A backlight source technology for an LED backlight source has been invented to overcome disadvantages of CCFL backlight sources, such as worse color restoration, low luminous efficiency, high discharge voltage, low discharge property in low temperature, long heating time for stable grayscale, etc. In a liquid crystal display, an LED backlight source and a liquid crystal display panel are set up in opposite for the LED backlight source to provide light to the liquid crystal display panel. The LED backlight source includes at least one LED string comprising a plurality of LEDs in serial. In a process of producing or assembling LED backlight sources, voltage applied on the LED string is over or under predetermined value on account of technical difference.
FIG. 1 is a circuit diagram of a driving circuit of an LED backlight source in a conventional LCD. As FIG. 1 shows, the driving circuit of the LED backlight source includes a voltage boost circuit 1, an LED string 21, a reference voltage module 41 and a voltage control module 3. The voltage control module 3 is coupled to a reference voltage VFB supplied by the reference voltage module 41, and the voltage control module 3 controls the voltage boost circuit 1 to boost an input voltage to a needed output voltage and to supply the needed output voltage to the LED string 21. The reference voltage module 41 comprises a resistor R1 and a resistor R2 connected in serial. The resistor R1 coupled between the voltage boost circuit 1 and the resistor R2. The resistor R2 is grounded. The reference voltage VFB is coupled to the resistor R1 and the resistor R2. In the circuit, a constant voltage fed to a positive end of the LED string 21 through the reference voltage module 41 is
      VFB    *                  R        ⁢                                  ⁢        1                    R        ⁢                                  ⁢        2              +      VFB    .  That is the circuit has an overvoltage protection (OVP) function with an OVP voltage value of
      VFB    *                  R        ⁢                                  ⁢        1                    R        ⁢                                  ⁢        2              +      VFB    .  Therefore, if the circuit is malfunction to make the actual voltage applied on the positive end of the LED string 21 exceeding the voltage needed, the circuit disables because the voltage fed to the positive end of the LED string 21 is
      VFB    *                  R        ⁢                                  ⁢        1                    R        ⁢                                  ⁢        2              +      VFB    .  However, the OVP voltage is incapable of adapting practical operation voltage of the LED string 21 in need when the rated voltage of the LED string 21 changes significantly.
For instance, the required operation voltage of the LED string is probably greater than the OVP voltage if the practical operation voltage of the LED string 21 in need is over to a predetermined voltage, which means the LED string needs greater voltage to work normally. It occurs that the LED string does not lighten. On the contrary, the required operation voltage of the LED string is probably less than the OVP voltage if the practical operation voltage of the LED string 21 in need is under to predetermined voltage, which means the LED string needs less voltage to work normally. If abnormal situation happens, it takes a long time to boost a voltage applied on the positive end of the LED to the OVP voltage so that it damages components.